Optical fiber fusion splicers commonly employ an electrical discharge to heat the fibers sufficiently for them to be fused together. This electrical discharge is known in the industry as an “arc”. However, according to some sources, a discharge of this current level is not a true arc, but a coronal discharge that generates a hot plasma field.
Recently, arcs of the same type have been adapted for use in stripping coatings from fibers and cleaning residual debris from mechanically stripped fibers. The arc is normally formed between sharply pointed tips of a pair of electrodes, spaced 1 mm to 10 mm apart, as shown in FIG. 1A. Larger electrode spacing is required for splicing multiple fibers at once (e.g., fiber ribbons), and for larger diameter fibers, as shown in FIG. 1B. The optical design of some splicers can also require the electrode spacing “gap” to be larger in order to prevent the electrodes from physically occluding the optical fiber path.
The electrodes are commonly made of tungsten. Although, in some cases, cerium or thorium are alloyed with the tungsten. These elements lower the thermionic work function of the electrode, which causes electrons to more readily leave the surface of the electrode. This allows the discharge to be initiated with a lower initial voltage. Alternatively, an external source of ions can be provided to assist in initiating the arc (e.g., Ion Enhanced Cold Plasma technology by 3SAE Technology, Inc.). It is possible to provide a suitable arc with ordinary steel electrodes and with no external ionization, but the repeatability of the arc characteristics is typically poor.
The voltage applied to the electrodes can be DC (typically in conjunction with smaller electrode spacing) or AC (which allows a larger spacing between the electrode tips—up to 10 mm or more). The voltage required to initiate the discharge is determined by Paschen's Law, which relates the breakdown voltage of a gap between electrodes to a (complex and non-linear) function of the gas present in the gap (e.g., typically ordinary air), pressure, humidity, electrode shape, electrode material, and gap distance. Many of the parameters required to apply Paschen's Law to this system are not known, so little quantitative theoretical analysis of splicer arcs has been done. Typically, the initiating voltage is determined experimentally to be in the range of 5 kV to 30 kV.
Once the arc has been initiated, sustained ionization of the plasma in the discharge requires a lower voltage than initially applied. The impedance (i.e., the ratio of applied voltage to current) of the plasma as a circuit element is difficult to predict. Splicer arcs are suspected to exhibit negative impedance at some frequencies and current levels. These characteristics make “constant voltage” operation of a splicer arc very difficult to achieve. Therefore, most of such systems are controlled to provide a constant average current. This correlates in a reasonably predictable way with the observed power delivered to the discharge and the resulting temperature of the fibers.
It is useful to provide a means of varying the arc power delivered to the fibers, in order to provide correct heating for different fiber types, and to compensate for differing conditions. This can be done by altering the current delivered to the sustained arc (with the control circuit mentioned above) or by pulsing the arc on and off.
Most common optical fibers are 80 μm to 125 μm in diameter (not including outer coatings), such as that shown in FIG. 1A. However, some applications, such as high-power fiber lasers, require fibers up to 1 mm or more in diameter. Most fusion splicers will not accept fibers greater than 200 μm in diameter. Specialty splicers exist for Large Diameter Fibers (LDF), with various maximum diameter capabilities, depending on design features.
Successful splicers for the larger end of the LDF (>600 μm) spectrum have typically used resistive filament heating or laser heating rather than an arc. For these large fibers, the dielectric nature of the fiber material can cause an arc to curve around the fiber, rather than enveloping the entire circumference of the fiber in the plasma field, as shown in FIG. 1B. This causes uneven heating of the fiber, with resulting poor splice quality.
Devices which use arcs to strip fibers can also suffer from uneven heating effects. These “arc strippers” typically place the fiber just outside the plasma field (above or below), so that heat from the arc causes decomposition of the coating. This necessarily causes the fiber to be hotter on one side than the other. For most coatings, this is not a problem. However, some coatings have a relatively narrow temperature window for effective removal and could benefit from more even heat distribution.